# term describing infrared trapping efficiency?

• posted on August 26, 2004, 5:44 am
does anybody know if there is a word or measurement that describes that efficiency at which a substance traps longer waved solar radiation?
For example glass is pretty good at trapping heat, so that may be rated at 100 "Scovillage" units or possibly it could be described as having a "Maxwell" rating of .5 celsius per second per cubic meter. Polycarbonate also used for green/glass houses may have a similar values while brick or other opaque materials would be close to zero. What would the real term for Scovillage or Maxwell be?

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• posted on August 26, 2004, 8:49 am
On Thu, 26 Aug 2004 05:44:55 GMT, Salty Thumb

Greenhouses? Child/pet endangerment units?

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• posted on August 26, 2004, 7:10 pm
If it what you want is truly a measure of efficiency, then it would be unit-less. (It's just a ratio)
For example:
The energy of one 680 nm photon is: E = hc/? 10-19 J
Photosynthesis requires 59 kJ per mol of ATP. Meaning that: EATP = 59 10 3 J mol-1 / 6.022 1023 mol-1 = 9.80 10-20 J per ATP molecule.
So, the energy efficiency of photosynthesis is: = 9.80 10 -20 J /2.92 10-19 J = 0.34 or 34%

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• posted on August 26, 2004, 2:40 pm
there is specific heat capacity http://scienceworld.wolfram.com/physics/HeatCapacity.html greenhouse gases http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html ability to absorb infrared and NOT re-emit it. CO2 and methane CH4 have very high absorption/retention rate way beyond ability of individual atoms. Ingrid

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• posted on August 26, 2004, 7:43 pm

I would be surprised if there were such a term because the greenhouse effect is not a property of the material alone. The obvious (to me, at least) variables are:     1) The spectrum of the incoming radiation. For an incandescent source such as the sun this is mostly determined by the source temperature. (The "surface" of the sun is about 6000 K) But atmospheric absorption figures in this as well.     2) The spectrum of outgoing radiation. This is mostly determined by the temperature reached by surfaces inside the greenhouse.     3) The transmission coefficient of the glass at every wavelength.
Also, it is not meaningful to talk about the greenhouse effect with opaque materials as there is no radiation passing through. (Unless you want to talk about x-rays and gamma rays.)
--
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A L B E R T A Alfred Falk snipped-for-privacy@arc.ab.ca

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• posted on August 26, 2004, 11:03 pm

I didn't like my chances, either, but I thought I'd ask anyway. To clarify, I'm looking for a term that enables comparisons of different materials for use in a greenhouse application. The term/measurement doesn't necessarily have to have mathematical precision, but if there is a directly measurable value that's good too. Something like R- values for building insulation in the US, or PR/+ ratings for microprocessors.
If the E in low E (emissivity) glass is quantifiable (even with some fudging allowed) and not just some vaccuous idea, then that's probably exactly what I'm looking for.
Seems polycarbonate is widely used, but is that because of superior trapping or a combination of trapping, structural strength, price and/or other?. I'd also like to see how Lexan, plain plexiglass, clear PVC and one other clear plastic (don't remember off hand) compare solely from a heat trapping / heat retention perspective.

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• posted on August 27, 2004, 11:17 am

I'm jumping back 30 years or so, but I think the ASHRAE Manual [American Society of Heating & Refrigeration mumble-mumble] has a table with material's emissivity and absorbtion rates. [and 'transfer rate' might be a separate table]
The only part of it I remember offhand is that unglazed brick is very similar to human flesh-- that's why we think of it as 'warm'.
The 'E' in 'E-glass' *is* quantifiable-- though I'm not sure if all manufacturers use the same standards yet. [they didn't in 1970] The emissivity is how much of the heat gets passed through-- not how much gets absorbed/trapped.
Jim